1. Field of the Invention
The invention relates to a fabric for a fibre channel which is one of data communication standards standardized as X3T11 by American National Standard for Information Systems (ANSI), and more particularly to a fibre channel fabric which is capable of higher speed operation and the greater number of nodes.
2. Description of the Related Art
A fibre channel, which is one of data communication standards standardized by ANSI as an I/O channel of computer system or communication medium for local area network (LAN), is characterized by making data communication at the unit of a variable length frame having 2148 bytes at maximum including header and other control data, having various flow control functions and a calling control function which is called class service, and being able to construct flexible communication system in line with various applications.
For instance, Ancor Communications-The World's Fastest Network, a version updated on Jun. 17, 1996, shows three topologies as illustrated in FIGS. 1, 2, and 3. Those three topologies are ones standardized as a fibre channel: a fabric type illustrated in FIG. 1; a point-to-point type illustrated in FIG. 2; and a loop type illustrated in FIG. 3. A fibre channel fabric acts as a data communication equipment which constitutes fabric topology of the above-mentioned fibre channels. Nodes which play a main role in data communication are connected to a fibre channel fabric in star-like arrangement.
A conventional fibre channel fabric interchanges a variable length frame as it is, namely, without converting a variable length frame into other forms, resulting in that a control thereof was unavoidable to be quite complicated, and that it was quite difficult or almost impossible to increase the number of nodes connecting thereto, and enhance interchangability thereof.
For instance, Japanese Unexamined Patent Publication No. 5-268255 has suggested a solution to a frame relay exchange system having the same problems as mentioned above. The Publication suggests dividing a variable length frame relay packet into fixed length ATM (Asynchronous Transfer Mode) cells, to thereby exchange cells for enhancement of interchangability. However, ATM characteristics make it difficult to avoid cell loss caused by congestion. In order to this problem, Japanese Unexamined Patent Publication No. 7-202903 has suggested a system where a frame relay packet is converted into cells by utilizing cell loss priority control identifier for ATM.
However, since calling setup is basically connection-oriented in a frame relay, a frame relay has high affinity with ATM. Accordingly, it is expected that the above-mentioned Publications can enhance interchangability, but the above-mentioned prior art is not applicable to a fibre channel, as mentioned below.
That is, a fibre channel has connection-oriented calling setup, which is called "class 1 service" in the fibre channel standard, and a connectionless calling setup, which is called "class 2 or 3 service" in the fibre channel standard. If the above-mentioned prior art were applied to the connectionless calling setup, a fabric would have to carry out ATM calling setup/releasing command each time when a communication of one frame of a fibre channel is made.
FIG. 4 illustrates a calling setup/releasing command sequence for B-ISDN (Broadband aspects of ISDN) which is typical communication network utilizing ATM and is recommended as Q.2931 by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector).
With reference to FIG. 4, in a system where ATM calling setup and releasing command are carried out each time before and after one frame having about 2000 bytes at maximum is interchanged, an overhead required for calling setup/releasing command is too excessive, resulting in reduction in performance greater than enhancement in interchangability brought by data communication utilizing fixed length cells. Accordingly, a fibre channel fabric needs to have a calling setup system which is capable of high speed operation and being constructed in simpler configuration, and which is not dependent on ATM calling setup system.
For the same reason, the above-mentioned prior art which attempts to avoid cell loss by virtue of ATM congestion control function is not applicable to a fibre channel fabric.
As having been mentioned so far, since the prior art fibre channel fabric interchanges a variable length frame as it is, namely without converting into other forms, the prior art fibre channel fabric is accompanied with problems that the control therefor is quite complicated, and that it is quite difficult or almost impossible to increase the number of nodes to be connected to the fibre channel fabric, and enhance interchangability thereof.
In addition, when a technique for enhancement in performance of a frame relay interchange system is to be applied to a fibre channel fabric, problems arise as follows.
First, a conventional system where a variable length data is interchanged by dividing into fixed length cells such as ATM has a problem that when a system for interchanging original variable length data is a connectionless one, an overhead concerned with calling setup is quite great, and hence interchangability is deteriorated to much degree. The reason is as follows. If original variable length data is connection-oriented, it would be possible to cause a time necessary for ATM calling setup to be contained in a time necessary for calling setup of the original variable length data, and it would be also possible to carry out data exchange after calling setup at higher speed by virtue of conversion into fixed length cells. On the other hand, in a system where calling setup is made frame by frame to a connectionless, variable length frame, a time necessary for calling setup is added to exchange time as an increase.
Secondly, the congestion control system suggested in a system where a frame relay packet is converted into cells requests a cell to be produced in accordance with ATM standards. Hence, if a cell were to be produced not in accordance with ATM standards, it was necessary to have another congestion control system in place of the above-mentioned one. The reason is that partial cell loss caused by congestion results that original variable length frame is unable to be reconstructed after cell exchanges, regardless of whether data is connection-oriented or not, and hence it is absolutely necessary to have any means for avoiding cell loss or means for detection and recovery.
Japanese Unexamined Patent Publication No. 62-155648, based on U.S. patent application Ser. No. 773,380 filed on Sep. 6, 1985 by Jonathan Seals Turner and assigned to Washington University, has suggested a packet switch apparatus, and a method of distributing a copy of data packet to a plurality of addresses.
Japanese Unexamined Patent Publication No. 7-321824, based on U.K. patent application No. 94303118.7 filed on Apr. 28, 1994 by Hewlett Packard Company, has suggested a chip used for cell switch fabric. The Publication suggests an integrated circuit chip which is applicable to cell switch fabrics having various structures and which is capable of interfacing a cell memory with N input and output ports.
Japanese Unexamined Patent Publication No. 6-335038, based on U.S. patent application Ser. No. 58,185 filed on May 10, 1993 by Lawrence Baranye and assigned to American Telephone and Telegraph Company, has suggested a method of replacing interchange fabrics in a data communication equipment.
Japanese Unexamined Patent Publication No. 61-501814, based on U.S. patent application Ser. No. 597,508 (PCT/US85/00557) filed on Apr. 6, 1984 by Royal M. Larthon et al. and assigned to American Telephone and Telegraph Company, has suggested multiplexed interconnection in packet interchange node package.
Japanese Unexamined Patent Publication No. 6-350651 has suggested hybrid type data processing apparatus which can process a packet signal, a frame relay signal, and ATM cell.
Japanese Unexamined Patent Publication No. 6-45944 has suggested an apparatus for encoding and decoding variable rate, which is capable of keeping quality in encoding and decoding data even if input data varies in an amount.
Japanese Unexamined Patent Publication No. 4-291548 has suggested a matrix type time sharing label interchange system where variable length and fixed length frames are contained in a common interchange device to thereby carry out uniform interchange.